The invention relates to a heat and creep resistant austenitic nickel chromium alloy steel such as is used in the petrochemical industry.
Such alloys require high strength, especially stress-rupture strength, and adequate toughness at the usual operating temperatures, as well as adequate resistance to corrosion.
U.S. patent specification 4 077 801 discloses a molybdenum- and cobalt-free austenitic cast nickel chromium alloy steel with 0.25% to 0.9% carbon, up to 3.5% silicon, up to 3.0% manganese, 8 to 62% nickel, 12 to 32% chromium, up to 2% niobium, 0.05 to less than 1.0% titanium, 0.05 to 2% tungsten and up to 0.3% nitrogen, balance iron, with high stress rupture strength and ductility at high temperatures. This cast alloy has good weldability and is a suitable material for apparatus for hydrogen reforming.
However, problems arise in view of the increasing process temperatures and the resulting reduction in life due to the decreasing creep strength with increasing temperatures and the fall in resistance to carburisation and oxidation.
The object of the invention is therefore to provide a nickel chromium alloy steel which can also withstand higher operating temperatures while having adequate creep strength together with resistance to carburisation and oxidation.
The achievement of this object is based on the concept of substantially improving the heat resistance of an austenitic nickel chromium alloy steel by means of cobalt and molybdenum together with certain intermetallic compounds. Cobalt improves the stability of the austenitic iron-nickel-chromium primary structure. This is the case particularly when the alloy contains ferrite-stabilising elements such as molybdenum for solid solution hardening.
In particular the invention consists in an austenitic alloy steel with 0.3 to 1.0% carbon, 0.2 to 2.5% silicon, up to 8% manganese, 30.0 to 48.0% nickel, 16.0 to 22.0% chromium, 0.5 to 18.0% cobalt, 1.5 to 4% molybdenum, 0.2 to 0.6% niobium, 0.1 to 0.5% titanium, 0.1 to 0.6% zirconium, 0.1 to 1.5% tantalum and 0.1 to 1.5% hafnium, the ratio of the contents of tantalum and hafnium to the zirconium content being more than 2.4%, and the total content of tantalum, hafnium and zirconium amounting to 1.2 to 3%. When its cobalt content is at least 10% the alloy steel contains more than 20% iron and when its cobalt content is less than. 10% it contains more than 30% iron.
The alloy has an austenitic iron-nickel-chromium or an austenitic iron-nickel-chromium-cobalt primary structure together with a high stress-rupture or creep strength and is resistant to both carburisation and oxidation. Nevertheless a further improvement in the stress-rupture strength is possible if at the expense of its essential constituents the alloy contains 1.5 to 2.5% aluminium and/or the contents of tantalum, hafnium and zirconium satisfy the following condition:
[(% Ta)+(% Hf)]/(% Zr)=1.2 to 14
A particularly satisfactory alloy is one with 0.42% carbon, 1.3% silicon, 0.40% manganese, 34.0% nickel, 19.0% chromium, 3.5% molybdenum, 0.40% niobium, 0.25% titanium, 0.30% zirconium, 0.15% tantalum and 0.80% hafnium, balance iron, or else one with 0.44% carbon, 1.2% silicon, 0.40% manganese, 33.0% nickel, 19.0% chromium, 3.0% molybdenum, 0.40% niobium, 0.20% titanium, 0.15% zirconium, 1.0% tantalum and 0.10% hafnium, balance iron.
Molybdenum improves the stress-rupture strength at intermediate temperatures, while intermetallic carbide phases impart to the iron-nickel-chromium primary structure, which in itself is weak, a high strength at temperatures up to 0.9 times its absolute melting point. Hafnium, zirconium, titanium, tantalum and niobium form primary carbides of the MC type, while chromium, in the presence of molybdenum, forms carbides of the M7C3 and M27C6 types in the intra- and interdendritic regions.